This invention relates to the field of semiconductor optical modulators and in particular to electro absorption modulators that are commonly used to provide external modulation to optical signals.
Optical transmission systems have seen dramatic increases in data transmission rates, with 10 Gb/s systems in use in many SDH networks, with 40 Gb/s systems under development. One technique that has been used to obtain such data transmission rates is external modulation of optical sources. Conventionally, optical sources such as laser diodes have been directly modulated by supplying the modulating signal to an electrode connected to the active region of the laser such that the output of the laser varies with the modulating signal. The main drawback with this technique is that the data transmission rates are limited by the photonic transitions that govern the population inversion and radiative decay. In comparison, external modulation relies upon an optical device that can be switched between an attenuating state and a substantially non-attenuating state such that data can be modulated onto the constant output of an optical source. One device that is commonly used to provide external modulation is an electro absorption modulator (EAM), the structure and operation of an example of an EAM is described in EP-B-0 143 000.
According to a first aspect of the invention there is provided an optical modulator comprising an input to receive an electrical input signal and an output to present an electrical output signal characterised in that the modulator additionally comprises control means to vary the electrical input signal in response to the electrical output signal. Preferably the optical modulator is an electro-absorption modulator. The electrical input signal may be a dc bias voltage. The electrical output signal may be an ac signal and preferably varies with the absorption within the electro-absorption modulator.
According to a second aspect of the invention there is provided an optical signal generator comprising an optical source and an optical modulator as described above. Preferably the optical source is a distributed feedback laser.
According to a third aspect of the invention there is provided a method of operating an optical modulator, the method comprising the steps of: (i) applying an electrical control signal to the modulator; (ii) applying an electrical modulation signal to modulate a received optical signal; (iii) receiving an electrical output signal from the modulator, the method being characterised by the step of (iv) varying the electrical control signal in response to the electrical output signal. Preferably the optical modulator is an electro-absorption modulator. The electrical control signal may be a dc bias voltage. The electrical output signal may be an ac current and preferably the electrical output signal varies with the absorption incurring within the modulator.
Preferably the electrical control signal is varied to increase the magnitude of the electrical output signal. The electrical control signal may be varied to maintain the magnitude of the electrical output signal at or above a predetermined threshold.